1. Field of the Invention
The present invention relates to master bases for fabrication, for example, a master base for manufacturing masters for the fabrication of microlens arrays, and a method for manufacturing the master base for fabrication.
2. Description of the Related Art
Currently, the following processing procedure is generally employed for manufacturing microlenses: a photoresist material is applied onto a substrate to form a photoresist layer thereon, the photoresist layer is patterned to form an array, and the obtained array is provided as a mold for microlenses.
In this processing procedure, as shown in FIG. 3, a single photoresist layer 20 is formed on a substrate 10, and the photoresist layer 20 is patterned to form a desired pattern by a photolithography method. However, there is a problem in that the obtained pattern does not have a smooth surface functioning as, for example, a lens surface. That is, steps 30 are situated on the surface of the pattern, as shown in FIG. 3B and FIG. 3C, which is a partly enlarged view of FIG. 3B.
It has been found that the above phenomenon is caused as follows: in a single photoresist layer on a substrate, rays applied to the photoresist layer interfere with rays reflected at an interface between the substrate surface and the photoresist layer surface to form standing waves and the photoresist layer is also exposed to the standing waves.
That is, when the single photoresist layer is exposed, the applied rays, which are supplied from a light source, interfere with the reflected rays to form other rays having different intensities. The formed rays having different intensities cause steps, which form the same pattern as the interference pattern, on the surface of the photoresist layer. As a result, the processed photoresist layer having an optical lens shape have steps on the surface, that is, the surface is not smooth. Therefore, when the processed photoresist layer is used as a master for manufacturing, for example, microlenses, obtained microlens have inferior converging performance due to the occurrence of scattering.
In the present invention, in order to solve the above problem, two or more photoresist layers are formed on a substrate to suppress or eliminate the adverse effects caused by rays reflected at the interface between the bottom of the photoresist layers and the substrate, as described below. However, this method employing two or more photoresist layers causes another problem. That is, in a step of forming a plurality of the photoresist layers, a photoresist layer is formed and is then pre-baked to form another photoresist layer thereon, and this procedure is repeated. In this procedure, when the formed lower photoresist layer is pre-baked at a temperature of less than 120° C., which is included in the pre-baking conditions of photoresist layers used in an ordinary photolithography method, the formed lower photoresist layer is not sufficiently cured. Therefore, when the upper photoresist layer is provided on the lower photoresist layer in this state to form a layered photoresist portion, the treatment of providing the upper photoresist layer causes the thickness of the lower photoresist layer to change so that the thickness of the entire layered photoresist portion changes. As a result, there is a problem in that the difference in thickness is increased depending on areas of the layered photoresist portion.
The difference in thickness causes a visible interference pattern over the upper surface of the layered photoresist portion.
FIG. 4 is an illustration showing a process in which the difference in thickness arises in the layered photoresist portion. As shown in FIG. 4A, a first photoresist layer 21 is formed on a substrate 10. As shown in FIG. 4B, the first photoresist layer 21 is then pre-baked at less than 120° C. When an second photoresist layer 22 is provided on the first photoresist layer 21 in this state, the flatness of the first photoresist layer 21 is damaged by the formation of the second photoresist layer 22, thereby causing waves over the top surface, as shown in FIG. 4C. This state causes the visible interference pattern described above.
If a master base prepared by the above method is used for manufacturing lenses, optical lenses having a desired performance cannot be obtained.